The present invention relates to a method and apparatus for judging road surface conditions and to a program for judging road surface conditions. More particularly, it relates to a method and apparatus for judging road surface conditions and to a program for judging road surface conditions with which it is possible to judge road surface conditions during running particularly in four-wheel-drive vehicles.
Antilock braking devices have been conventionally proposed in which locked conditions of wheels are prevented by decreasing braking torque acting on the wheels prior to occurrence of a condition in which the tires are locked with the braking force between tires and the road surface exceeding a maximum value to thereby control the rotational number of the wheels with which maximum braking force can be obtained. In such an antilock braking device, it is possible to employ a method for judging coefficients of road surface friction in which, for instance, a relational expression between an acceleration of the vehicle and a slip ratio is obtained whereupon a slope of this relational expression is compared with a preliminarily set threshold, and the coefficient of friction between the road surface and tires is judged on the basis of the result of this comparison (see Japanese Unexamined Patent Publication No. 2001-334920).
However, in a relationship between the acceleration of the vehicle and the slip ratio in such a method for judging coefficients of road surface friction, a gradient of μ-s curve of the road surface and the tires is obtained on the basis of the acceleration of the vehicle obtained from a rotational velocity of the following wheels and a tendency of slip of the road surface is judged accordingly so that a drawback is presented that this method is not applicable to four-wheel-drive vehicles.